A draw frame of the type referred to has been disclosed in commonly owned U.S. Pat. No. 4,314,388, for example. According to that prior patent, a plurality of cascaded drawing stages are constituted by respective roller pairs, each including a driven lower roller and a set of corotating counterrollers which may be regarded as a single upper roller. The lower rollers are rotated by synchronous motors at speeds determined by respective digital frequency selectors energized from a common source of three-phase current. Such frequency selectors are also disclosed in commonly owned U.S. Pat. No. 4,336,684.
When incoming card sliver to be doubled or plied in such a draw frame is combined into an outgoing fiber bundle, the latter often lacks the necessary uniformity until various changes have been made in the operating parameters which determine the tension imparted to the fibers and the resulting bulk or thickness of the outgoing bundle. Once the proper speed ratio has been established among the several roller pairs, the setting of the associated frequency selectors can be left unchanged upon a switchover to a different fiber assortment even if the absolute roller speeds are to be modified. The use of an optimum speed ratio, however, does not by itself eliminate thickness fluctuations of the resulting fiber bundle.
While speed ratios can be precalculated, other parameters affecting the uniformity of the fiber bundle can be optimized only by trial and error. These parameters include the effective spacing of the roller pairs from one another, referred to hereinafter as their nip-line distance, and the contact pressure exerted by the rollers of each pair upon the fibers clamped therebetween. Changing the nip-line distance modifies the tensile stress imparted to the fibers while a variation of the clamping force alters their compressive stress. Making such changes by hand, e.g. in an initial phase of a new fiber-drawing operation, is a laborious and time-consuming task.